1. Field of the Invention
The present invention relates to the field of radio communications. More particularly, the present invention relates to a radio communication system, a radio transmitter and a radio receiver that adopt the OFDM (Orthogonal Frequency Division Multiplexing) scheme.
2. Description of the Related Art
The OFDM scheme, which is getting attention in the field of radio communications, is a technology for realizing good signal transmission even under a multipath transmission environment by arranging plural carriers (sub-carriers) at intervals of a frequency such that the plural carriers are orthogonal to each other. In a transmitter in this scheme, as shown in FIG. 1, a signal is generated in a signal generation part based on transmission information. The signal is converted to a parallel signal series by a serial-parallel conversion part (S/P), processed by an inverse fast Fourier transform part (IFFT part), and converted to a serial signal series by a parallel-serial conversion part (P/S). Then, a guard interval is added in a guard interval adding part (GI), and the signal is amplified by a power amplifying part (PA) so as to be transmitted as an OFDM signal by radio. The signal generation part performs error-correction encoding, interleaving and symbol mapping and the like so as to generate a transmission symbol. As is generally known, the guard interval is a copy of an end part of the transmission symbol. On the other hand, in the receiver, as shown in FIG. 2, the guard interval is removed from a received signal by a guard interval removing part, the received signal is converted to parallel signals by a serial-parallel conversion part (S/P), the parallel signals are demodulated by a fast Fourier transform part (FFT part), and further demodulated in a signal detection part so that the transmission information is obtained. The signal detection part performs channel compensation, symbol mapping, error-correction decoding and the like on the basis of a channel estimation result.
In the OFDM scheme, since various sub-carriers are used, a transmission signal with a very large amplitude may be generated in some cases as shown in FIG. 3. The ratio of a possible maximum peak power to an average power is referred to as PAPR (Peak to Average Power Ratio). Generally, the maximum peak power may become the number of all sub-carriers times larger than average power.
On the other hand, as shown in FIG. 4, the power amplifier (PA) has a linear region that provides linear input/output characteristics and a non-linear region that provides non-linear input/output characteristics. For outputting a transmission signal having small distortion, it is desirable that the power amplifier (PA) operate in the linear region. If the power amplifier (PA) operates in the non-linear region, there may arise problems such as degraded transmission quality, unwanted emission to outside of the band. When the PAPR is large, the power amplifier is being used not only in the linear region but also in the non-linear region. A power amplifier having a wide liner region sacrifices amplifying efficiency. Therefore, it is desirable that the PAPR of the transmission signal be small.
In one document (X. Li and L. J. Cimini, “Effects of clipping and filtering on the performance of OFDM”, IEEE Commun. Lett., vol. 2, no. 5, pp. 131-133, May. 1998), a large peak value is clipped for reducing PAPR.
In another document (M. Friese, “On the degradation of OFDM-signals due to peak-clipping in optimally predistorted power amplifiers, Proc. of GLOBCOM '98, pp. 939-944, November 1998), a so-called pre-distortion scheme is adopted, in which inverse characteristics of the distortion are weighted on a signal input to an amplifier, which enables to amplify the signal linearly if it is below the saturation level.
However, the pre-distortion scheme clips a part of a signal having a peak power level exceeding a saturation region of the amplifier.
In the method in which an unwanted peak power value is clipped, orthogonality among sub-carriers collapses, so that interference between symbols increases and receiving quality may be degraded.